Post-treatment of laminated nonwoven cellulosic fiber webs

ABSTRACT

A method for post-treating a laminated precursor nonwoven web which includes layers of thermoplastic man-made fibers and at least one layer of cellulose-based staple natural fibers, including consolidating the web laterally and thereby reducing the maximum pore size measure of the web. The precursor web and the resultant consolidated nonwoven web are also disclosed, as is utilization of the product web in medical uses.

This patent application is a continuation application of Ser. No.08/037,228, filed Mar. 26, 1993, now abandoned, which is acontinuation-in-part application of Ser. No. 07/858,182, filed Mar. 26,1992 and which issued as U.S. Pat. No. 5,244,482 on Sep. 14, 1993.

FIELD OF THE INVENTION

This invention relates generally to laminated webs made up of at leastone inner layer of cellulosic material sandwiched between outer nonwovenlayers, which webs have been post-treated to reduce the pore size in theweb and/or to impart other desirable properties to the web. In oneaspect, the invention relates to post-treatment of a laminated precursorweb to improve the web's properties for a variety of uses. In anotheraspect of the invention, nonwoven webs of man-made thermoplastic fibersare firstly laminated with at least one inner layer of cellulosicfibers, secondly drawn under thermal conditions, and thirdlymechanically compacted to efficiently alter the geometric arrangement ofthe fibers making up the web resulting in web having reduced measures ofpore size and/or other desirable properties.

BACKGROUND OF THE INVENTION

As indicated above, the present invention relates to the post-treatmentof laminated webs which include outer nonwoven non-elastomeric layersbetween which there is sandwiched a layer of cellulosic fibers to alterthe filament spacing and structure of at least the nonwovennon-elastomeric webs. The terms "web" and "layer" are used hereininterchangeably at times, the distinction therebetween being obviousfrom the context in which the terms are used In either event, "web" and"layer" imply a self-supporting planar member formed of fibers orfilaments as the case may be. It should be also observed that the terms"filaments" or "fibers" are used interchangeably herein, although"fibers" in nonwovens generally refers to discontinuous strands and"filaments" as continuous strands. The present invention contemplateswebs with continuous synthetic thermoplastic filaments and/ordiscontinuous fibers in the non-elastomeric nonwoven layers of man-madefibers.

In the present invention, a precursor web made up of an inner layer ofstaple-length cellulosic fibers which is sandwiched between outer layersof man-made fibers, i.e. synthetic, thermoplastic, nonelastomericfibers, is consolidated by heating and drawing in the machine direction(MD) to provide a consolidated web which has reduced measures of poresize and enhanced breathability, strength, hand, absorbent capacity,wicking and barrier properties. The layers of man-made fibers preferablyare formed by meltblowing or spunbonding techniques. Meltblown fibers ofthese man-made fibers preferably are of a diameter of between about 0.5and about 10.0 micrometers; whereas, the diameters of the fibers inspunbond webs overlap with meltblown webs on the low end at about 8.0micrometers and may range up to 50 micrometers or more on the upper endof their diameter range. Spunbond webs generally are coarser butstronger than meltblown webs because spunbond fibers are given notableorientation after quenching. In either instance, the fibers are formedinto self-sustaining webs. The preferred web weight of a meltblown webfor use in the present invention is light weight, having a weight in therange of between about 0.05 and about 10 oz/yd², and most preferablybetween about 0.25 and about 2 oz/yd². The preferred weight of aspunbonded web for use in the present invention is also light weighthaving a weight between about 0.1 and about 10 oz/yd² and mostpreferably between about 0.3 and about 2 oz/yd². Webs of weights lighterthan about 0.05 oz/yd² tend to be of insufficient fiber density forcontaining the cellulosic fibers and providing the strength and otherproperties desired in the composite web. The heavier weight webs, i.e.above about 10 oz/yd² tend to develop undesirably harsh composite webswhen combined with the cellulosic fiber layer. More specificdescriptions of the spunbonding and meltblowing processes, and the websso produced are given in the publication entitled: "Proceedings, FiberProducer Conference 1983", Apr. 12, 13 & 14, 1983, pp. 6-1 through 6-11,such publication being incorporated herein by reference.

Since the development of the meltblowing process by the Naval ResearchLaboratory in 1951 (published in 1954 by the U.S. Department of Commercein an article entitled "MANUFACTURE OF SUPERFINE ORGANIC FIBERS"), whichpublication is incorporated herein by reference, there has been aconsiderable effort by several companies operating in the industry tofind new uses for the nonwoven product having microsized fibers. Becauseof the random, geometric assembly or structure of the fibers, andrelatively small fiber size, the fibers have received extensive use asfilters. Further and/or different uses of these meltblown webs isdesired.

In the formation process for most random laid or unordered fibrous webs,the pore size that develops is directly related to the square of thefiber diameter. The spunbonded process is distinguished from meltblowingby self-bonding and non uniform draw down (plastic deformation) offilaments forming the web. Thus meltblown webs have a relatively broaddistribution of fiber diameters. Typical nonwoven webs produced bymeltblowing have fiber diameters of 0.5 to 20 microns, preferably 0.5 to8 microns, making them suitable for filtering out 5 micron particles at80 percent efficiency or greater. It is known that filtration can beimproved by practicing the web formation process to produce smaller andsmaller diameter fibers while controlling other formation parameterssuch as porosity and thickness. As noted above, this results in smallerpore size thereby improving the efficiency of particle removal infiltration. By operating the meltblowing process under extremeconditions, the fiber size can be produced in the order of 0.1 to 5microns. The process, however, has the following disadvantages: lowproduction rates, high energy usage. In order to improve the propertiesof the nonwoven web, efforts have been made to post-treat the webs by avariety of processes. Such efforts have included post calendering theweb to improve, the tensile strength of the web and, postelectrification as disclosed in U.S. Pat. No. 4,592,815 to improvefiltration performance of the web, to name but two of such efforts. Itis significant to note that none of these prior art techniques have beendirected specifically at planar consolidation to reduce the size of thepores in the web. Neither is it known to attempt consolidation oflaminates of these webs, particularly when laminated with disparatefibrous layers such as layers of cellulosic fibers.

Calendering of nonwovens flattens fibers and consolidates the web in adirection normal to the plane of the web and reduces the thickness.This, however, leads to reduction in permeability which is an importantproperty to conserve for many purposes such as breathability andfiltration. U.S. Pat. No. 4,048,364 discloses a process for drawing themeltblown web in the machine direction (MD) to produce a ten-foldincrease in the tensile strength of the post-drawn web. It issignificant to note, however, that the precursor web required in thisinvention contains relatively coarse fibers (10 to about 40 micronsaverage fiber diameter) and polymer of low crystallinity. Lowcrystallinity generally means about 22% or less. The extensive drawingof the web reduces the diameter of the fibers in the machine directionto an average diameter of 1 to 8 microns at draw ratios ranging from 2:1to 10:1 and preferably 5:1 to 7:1. The main purpose of the process is toincrease the molecular orientation to enhance the strength of thegreatly drawn fibers. Precursor webs of very high post processing drawratio capability are required in order to prevent rupture of fibers inthe web drawing process. Tests have shown that the stretching of aprecursor web having hot (e.g., 10° F. less than the melting point ofthe precursor web) drawing capabilities from about 5:1 to 10:1 does notalter the measure of pore size of the web. This is probably due to thefact that the high and easy drawability of the fibers prevents thedevelopment of sufficient, compressive forces to bend the stout fibersin the web and physically reduce its pore dimensions and measures ofpore size distribution in general.

Nonwoven webs (fabrics) are defined as "sheet or web structures made bybonding and/or interlocking fibers, yarns or filaments by mechanical,thermal, chemical or solvent means." These webs do not require theconversion of fibers to yarn. Nonwoven webs are also called bonded orengineered webs and are manufactured by processes other than spinning,weaving or knitting, hence the name "nonwovens". The fibers of anonwoven web are substantially randomly laid to form a web wherein someof the fibers are bonded by fiber-to-fiber fusion, or fiberentanglement, or thermal bonds as by point bonding. The basic structureof all nonwovens is a web of fibers or filaments. A single type of fiberor filament may be the basic element of a nonwoven. Fibers that aremeasured in a few centimeters or inches or fractions thereof are calledstaple fibers. Those fibers of extreme length are called filamentfibers. In general filament fibers are measured in terms of kilometersor miles. In fact, filament fibers are not readily measured, as they maybe many, many kilometers in length. In fibers, the length must beconsiderably greater than the diameter, e.g., a length-to-width(diameter) ratio of at least 100 and usually considerably higher. Cottonfibers may measure from less than 1/2 inch to more than 2 inches inlength and have a typical length-to-diameter ratio of about 1400. Othernatural fibers exhibit typical ratios as follows: flax--1200;ramie--3000; and wool--3000. In the present application, the terms"fiber" or "fibers" are intended to include both short and long fibers,i.e. staple fibers and filament fibers, unless otherwise specificallyindicated by identifying the fibers as staple or filament. For example,spunbonded webs are formed of filament fibers, whereas meltblown websinclude an assortment of fiber lengths so that these webs commonlyinclude both staple length and filament length fibers. In nonwovens, theindividual fibers may be in an organized or in a random arrangement.Tensile, elongation, and hand properties are imparted to the web by thetype or types of bonding as well as fiber-to-fiber cohesion andreinforcement by its constituents. The technology for making nonwovenwebs is based on the following primary elements: fibers of variouslengths and diameters; a web arranged according to the method of formingand processing; the bonding of fibers within the web and reinforcementby its constituents. The variation of one or several elements incombination allows for the enormous range of nonwoven fiber types.Control of the type and length of the fibers and of the bonding, incombination with the selection of the manufacturing method, gives riseto a highly technical, yet extremely flexible combination of options.

Nonwoven webs of man-made, i.e. synthetic polymeric, fibers (asdistinguished from "natural" fibers such as cotton, ramie, wool, etc.)have heretofore found acceptance in the medical industry as disposablesubstitutes for the prior art reusable cotton examination gowns,surgical gowns, surgical drapes, face masks, shoe covers, sterilizationwrap and other products, to the extent that this market for nonwovenproducts is estimated to exceed one billion dollars annually. Further,nonwoven webs have found use in sanitary products, such as sanitarynapkins, disposable diapers, incontinent pads and other similarproducts. One of the benefits of nonwoven webs heretofore has been theirrelatively low cost, as compared to woven webs. The difference in costbetween nonwoven and woven webs has heretofore been of a magnitude suchthat the end users can dispose of the nonwoven web product after asingle use and yet realize a monetary gain over the multi-use wovenwebs.

Among the desired properties of a nonwoven web for use in medical andsanitary applications are the hand (softness and drapability), wicking,liquid retention, absorptive capacity and strength of the web. Also ofimportance in acceptance of the nonwoven web by the end user is thedegree to which the nonwoven web approximates the desirable propertiesof the woven webs, in particular woven cotton webs. Nonwoven webs ofman-made fibers generally have the reputation of notoriously lackingmany of the properties of woven natural-fiber webs, in particular hand,wicking, and liquid absorption and retention. Meltblown nonwoven webs,for example, exhibit a void volume of about 85%; spunbonded nonwovenwebs exhibit a void volume of between about 90 and 95%. These webs,further, often exhibit undesirable chemical properties, such ashydrophobicity, that make the webs less than desirable for use inmedical applications, for example. Moreover, the surface properties ofthese nonwoven webs tend to be smooth, hence exhibit a slick or oilyfeel and appearance. The man-made fibrous material of the prior artnonwoven webs most commonly exhibits a low surface tension so thataqueous liquids are not attracted thereto so that these prior art webshave poor wicking and retention of these liquids. These webs also aredifficult to treat with liquid repellents. Still further, thefilamentary nature of the man-made fibers of many prior art webs andtheir methods of manufacture cause the fibers to lay in the webs withthe length dimension of the fibers oriented substantially parallel tothe plane of the web so that the webs have poor absorbency of liquidsinto the body of the web. Considerable effort has been exertedheretofore to improve these properties of nonwoven webs, includingmodification of the manner of manufacturing and/or processing the web.These efforts, however, increase the cost of the nonwoven web and mayadversely alter its monetary advantage over woven webs of naturalfibers. Further, the man-made fibers of nonwoven webs most commonly arepetroleum-based and therefore have been subject to the substantialfluctuations in market price of this raw material, and the importantconsiderations in ultimate disposal of the product after use.

SUMMARY OF THE INVENTION

It has surprisingly been discovered that by selecting a precursorlaminated nonwoven web with certain properties and post-drawing the webunder certain conditions, at least the synthetic man-made fibers of theprecursor web are restructured to provide the laminated web with uniquemeasures of pore size, directional absorption, elastic recoveryproperties, strength, wicking, liquid absorption capacity, breathabilityand barrier properties, as well as good drape and hand which make themideally suited for a variety of end use applications such as protectiveapparel, face masks, diapers or sanitary napkin parts, wound dressings,respirators, wipes, chemical reservoirs, wicks, and surgical drapes.

In accordance with one aspect of the present invention there is provideda novel multilayered precursor web; all of the layers of whichpreferably are nonwoven, and which exhibits the desirable properties ofa woven web of natural fibers and the economic advantages of a nonwovenweb of man-made fibers. The precursor web of the present invention ismultilayered and comprises a first layer of nonelastomeric, man-madefibrous material selected from the group consisting of thermoplasticmeltblown man-made fibers, thermoplastic spunbonded man-made fibers,thermoplastic man-made staple fibers, and combinations thereof, thisfirst layer being light weight and having a weight of between about 0.05and about 10 oz/yd², and a second layer of cellulose-based naturalstaple fibers, excluding wood fibers, and having a weight of betweenabout 0.1 and about 10 oz/yd², the fibers of the second layer having afiber length of between about 0.5 and about 3.0 inches and a fineness ofequivalent to between about 3 and 5 Micronaire units. In a preferredembodiment, the precursor web includes at least a third layer ofnonelastomeric man-made fibrous material selected from the groupconsisting of thermoplastic meltblown man-made fibers, thermoplasticspunbonded man-made fibers, thermoplastic man-made staple fibers andcombinations thereof. This third layer preferably also is light weightand has a weight of between about 0.05 and about 10 oz/yd², and isdisposed on that side of the second layer opposite the first layer andthermally bonded to at least the second layer such that the second layeris sandwiched between the first and third layers. Other and additionallike layers of like materials may be included in the laminate. Thelayers are preferably thermally bonded together to form a coherent web,the area of bonding between the layers being between about 5 and about75% of the area of one of the flat surfaces of the laminated web. Thebonding contemplated in the precursor web is of a type which does notadversely affect the hand and other physical characteristics of theproduct web such as liquid wicking and absorbent capacity. Accordingly,the preferred bonding is effected from only one side of the laminate.

The laminated precursor web of the present invention, regardless of thenumber of layers employed in its makeup, preferably exhibits a finalcomposite weight of between about 0.5 and about 24 oz/yd² in order toapproximate a woven web of natural fibers in feel, drapability and otherproperties. This limitation upon the present web requires that there becareful selection of the weight of each of the individual layers of theprecursor web which will provide other desirable or required propertiessuch as strength, wicking, liquid absorption and retention, and barrierproperties (ability to exclude liquids while permitting or evenencouraging vapor and gas transfer through the thickness of the web).This laminated precursor web is thereafter consolidated as describedherein.

The method of the present invention involves subjecting the bondedlayers of the precursor web, especially the layers of man-madethermoplastic fibers or filaments, which layers have relatively lowtensile extensibility during post processing (as reflected by a low drawratio at break), to primary drawing under an elevated temperature. Thisuni-directional drawing in the MD laterally consolidates the precursorweb to a great extent thereby reducing both the average pore size of theweb and narrowing the pore size distribution, as well as imparting tothe web the further unexpected, but desirable properties of strength,wicking, liquid absorbent capacity, breathability and barrierproperties. The resultant consolidated web exhibits improved uniformityin pore size and high lateral elasticity characteristic of "stretchfabric" having approximately 120% elongation to break. Further theresultant web, even though of improved breathability, exhibits goodbarrier properties, such as being resistant to strike-through of liquidsby reason of the properties imparted to the consolidated web by thelayer of natural staple fibers. Thus, the composite web of the presentinvention is particularly useful in the manufacture of disposablemedical products because of its superior barrier properties, hand,breathability, strength, wicking and liquid absorption and retention,among other properties.

In an alternate embodiment, the web being drawn may be passed intosupplemental mechanical compacting means to induce and/or refine theprimary web consolidation.

Although the present invention is described and exemplified inconnection with meltblown and spunbond webs, it is to be understood thatit has application with other nonwovens of man-made fibers such ashydro-entangled, needled webs, and laminated combinations of these andwith other web forms such as air laid, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of apparatus for producing meltblown webs.

FIG. 2 is a perspective view of apparatus for the practice of webconsolidation of the present invention.

FIG. 3 is a perspective view of an alternate embodiment of an apparatusfor the practice of web consolidation of the invention illustrating thedrawn web passing over a torus surface for variably imparting compactionforces to the consolidating web.

FIG. 4 is an enlarged plan view of a tiny planar segment of a meltblownweb illustrating the random nature of a layer of man-made fibers of aprecursor web useable in the present invention.

FIG. 5 is an idealized plan view representation of the fibers of a layerof man-made fibers of a precursor web facilitating the analysis of themechanisms involved in the present invention.

FIG. 6 is a view similar to FIG. 5 after the web had been drawn.

FIG. 7 presents two curves illustrating the pore size distribution of alayer of man-made fibers of a web before and after drawing.

FIG. 8 is a plot illustrating that precursor meltblown webs (circles)having average fiber diameter less than eight microns (sample data fromTables I and II) are increasingly densified by the post-drawing(squares).

FIG. 9 is a plot illustrating that precursor meltblown webs (circles)having fiber diameter greater than about eight microns show negligibleimprovement in particle filtration efficiency after post drawing(squares).

FIG. 10 is a schematic representation of one embodiment of a laminatedprecursor web which incorporates various of the features of the presentinvention;

FIG. 11 is a schematic representation of a process for the formation ofa laminated precursor web which incorporates various of the features ofthe present invention;

FIG. 12 is a schematic representation of a further process for themanufacture of a laminated precursor web which incorporates various ofthe features of the present invention; and,

FIG. 13 is a schematic representation of a still further process for themanufacture of a laminated precursor web and depicting in-lineweb-forming apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As indicated above, the present invention relates to the post-treatmentof a laminated precursor nonwoven web to reconstitute or restructure thefibers of the web, to reduce the measures of pore size and impart to theweb other beneficial properties. The term "pore size" means aquantification of the physical dimensions of channels oriented in agenerally normal direction to the plane of the web. The pore size valuesrecited herein are based on standard test method ASTM F 316-86.

The present invention described with specific reference to the preferredwebs will be meltblown webs; it is to be emphasized, however, that themethod and product produced thereby includes other nonwoven webs ofman-made fibers, specifically spunbond, hydro-entangled, needled websand laminated combinations of these. Also the consolidated web producedaccording to the present invention may be used in combination with otherwebs or substrates such as webs from elastomeric polymers, microporousfilms or stretch limiting materials post laminated to limit the CDextensibility to less than 100% and provide additional performanceproperties for added utility.

Meltblowing is a well known process which generally utilizes equipmentdepicted in the schematic of FIG. 1. The process is carried out byintroducing a thermoplastic resin into an extruder 10 where the polymeris heated, melted, and extruded through a die 11 to form a plurality ofside-by-side filaments 12 while converging layers of hot air(discharging from slots 13 on each side of the row of filaments) contactthe filaments and through drag forces stretch and attenuate thefilaments 12 to a micron-size. The fibers 12 are collected onto acollector such as a rotating screen 15 forming a nonwoven web 17 whichmay be withdrawn on a take-up roller for later processing. The collector15 may include a vacuum screen wherein a vacuum, through line 18, isdrawn by a vacuum pump 19.

The hot air (primary jet air) is introduced into opposite sides of thedie through line 14. Although not indicated on the drawing, secondaryair which is aspirated into the primary air/fibrous stream serves tocool the filaments discharging from the die 11.

The process and apparatus described above forms no part of the presentinvention; however, variables used in the process, (including the typeof resin employed, the amount and temperature of primary air and polymermelt, and the spacing of the collector 15 from the die discharge) willhave a significant effect on the precursor web properties.

Briefly, the process in one embodiment of the present inventioncomprises the steps of (a) selecting a laminated nonwoven precursor webmade up of multiple layers including at least one layer of staple-lengthcellulosic natural fibers sandwiched between at least two layers, atleast one of which is made up of synthetic, thermoplastic nonelastomericfibers or filaments, the layers being bonded into a coherent websuitable for consolidation in the manner described herein, the laminatedprecursor web having substantial fiber bonding and having relatively lowprocessing extensibility, and (b) passing the nonwoven laminatedprecursor web through a heated zone to increase the temperature of theweb to the softening temperature of one or more of the thermoplasticcomponents thereof while drawing the web in the machine direction (MD)thereby greatly plastically bending the cross direction (CD) ofsynthetic thermoplastic fibers in the web which consolidates the web inthe CD reducing the maximum pore size of the precursor web by at least20 percent, and, more significantly, reducing the pore size distributionby at least 20% and imparting to the web enhanced strength,breathability, wicking, liquid absorption capacity and barrierproperties. As described in detail below, the precursor web must havecertain properties to enhance consolidation.

Apparatus for carrying out a preferred consolidation process isillustrated schematically in FIG. 2 wherein the precursor web 17 isunwound from roll 20 and fed through the nip of counter-rotating feedrollers 22, through oven 23, and finally through the nip ofcounter-rotating rollers 24. The oven 23 is maintained at a temperatureto heat the precursor web 17 to a temperature between its softeningpoint and the melting point of the polymers in the web. Preferably theweb is heated to a temperature within 10° to 15° F. of the melting pointof at least one of the thermoplastic components of the web. The rotatingrollers 24 are driven at a speed in excess of the rotating feed rollers22 so that the output velocity (V2) of the web is in excess of the feedvelocity (V1) for the draw ratio which is a function of the velocityratio V2/V1. The initial drawing of the web 17 under thermal conditionscauses the web to contract within the oven 23 from its feed width 17a asillustrated by web section 17b in FIG. 2. This contraction is dueprimarily to the plastic bending deformation by planar compression ofgenerally CD thermoplastic fibers of the web thereby reducing themeasures of pore size of the web. It is important to note that the highMD tensile forces developed at low MD strain during drawing, togetherwith the network nature of the fiber-fiber bonds in the web augments thegeneration of enough compressive stress to easily bend most CDthermoplastic fiber segments 27 and compact the web in the CD as shownin FIG. 6. Since fiber bending rigidity of the thermoplastic fibers isrelated to the fourth power of the fiber diameter, only webs havingsmall average fiber diameters can be consolidated by the availablestresses with the associated reduction in pore size measures. Averagefiber diameter for meltblown webs are preferably less than about 9microns, and less than about 50 microns for spunbonded webs.

The lateral contraction which results in pore size reduction is notaccompanied by significant average fiber diameter reduction of MDfibers. Continued web stretching beyond that necessary for web pore sizereduction may cause fiber diameter reductions. The web is contracted toa minimum width 17c as the web 17 exits the oven 23 or as the web 17passes the nip of rollers 24. It is preferred, but not essential, tocool or permit the web to cool between the exit of the oven 23 and thenip of the rollers 24 thereby controlling the heat set or annealing inthe restructured fibers under stress.

As the web 17 cools to between 130° and 90° C. (for PP), the web can beelectrostatically charged to impart durable enhanced filtrationefficiency to the web products. (The nip of the rollers 24 and that ofrollers 22 preferably are parallel so that the tensile force applied byrollers 24 and the resistance applied by rollers 22 are uni-directional[e.g., uniaxial]).

To further control or narrow the distribution of pore sizes,supplementary or alternative web-width compaction means can be addedbetween 17a and 17c as schematically illustrated in FIG. 3. FIG. 3 showsone alternate web processing embodiment in which the web passes into asupplementary or alternative web compacting device consisting of a(tilted) section of a torus 25. The consolidation interval of the web 17and the torus bar 25 are heated in an oven or heated to provide theproper temperatures for drawing and consolidating the web. The webenters the outboard surface (of diameter D) of the torus at widthdimension 17d and exits near the inboard surface of the torus which hasa lesser width dimension 17e. The converging surface of the path aroundthe torus applies transverse compressive forces in the plane of the webof entry width 17d. The added compressive forces overcome the bendingresistance of inefficiently deformed large CD fiber segments or shotimperfections remaining in the web 17 following primary consolidation(if used). This improves the uniformity in pore sizes. The heating andstretching of the apparatus in FIG. 2 (gross drawing) and FIG. 3(secondary drawing) can be carried out in series. The primaryheating-drawing step imparts gross consolidation while the secondarytorus consolidator refines the processing. The maximum compressivestrain imparted to the web by traversing about 180° around the torussurface is given by (D-d)/D, where D is the outboard or entry perimeterrelated to the entry width 17d and d is the inboard or web exitperimeter of the torus 25. The magnitude of the supplementaryconsolidation can be adjusted by adjusting the two diameters of thetorus 25 compacting device or "c-roll" shown in FIG. 3. If the c-roll ismade straight (viz. radii=∞), then no lateral compaction occurs and theroll solely increases the anneal time and maintains the thickness of theweb. The torus surface can be fixed or can be a rotatable curvedflexible bar. A fixed torus 25 with an air bearing between the surfaceand the web allows high lateral compressive strain and low friction foradditional MD draw. It should be noted that revolving "Bowed rolls" areonly used in textile applications to remove wrinkles from a movingtextile fabric by laterally stretching the fabric as the textileproceeds around a surface of diverging width.

The physical properties of the precursor web are basically determined bythe layers of synthetic thermoplastic nonelastomeric man-made fibers.The layer of staple-length cellulosic natural fibers is of low strength,relative to the strength properties of the sandwiching layers ofthermoplastic fibers, so that the contribution of the cellulosic layerto the physical properties such as the high MD tensile strength of theman-made fibers and the bending rigidity of the CD man-made fibers whichare looked to in determining the acceptability of a precursor web forconsolidation, is of essentially no effect. The nonelastomeric nonwovenman-made fibrous layers of the precursor web are selected based on theirdimensions, and hot processing tensile properties (viz.,elongation-at-break). In general, the breaking draw ratio of theprecursor web during hot processing should be less than about 4.0 andgreater than about 1.4 evaluated while hot drawing at a strain rategreater than 2500%/min and temperature greater than the softening pointbut at least 10 degrees F. less than the polymer melting temperature.This is an important indicator of precursor molecular orientation statefor achieving sufficient stresses for CD thermoplastic fiber bucklingand bending to cause reduction of the measures of pore size distributionof the web by the process of the present invention. The room temperatureelongation (strain) at break should be between 2 and 40 percent,preferably between 5 and 20 percent, based on test method ASTM D 1117-77using the Instron tensile testing machine. Note that the precursor websdisclosed in U.S. Pat. No. 4,048,364 are totally unsatisfactory for usein the present invention because such precursor webs are characterizedas having at least 50%, preferably at least 70%, standardized elongationbefore break, preferable max processing draw ratio greater than 5. Websmade up of low modulus, low crystalline (less than 22%), exhibit toomuch elongation at low tension in the heating and drawing step to permitdevelopment of the necessary stresses. The webs useful in the process ofU.S. Pat. No. 4,048,364 have far greater maximum draw ratio than 4 underthe hot draw condition described above. It is estimated that these drawratios are greater than 5.

Compressive stresses which buckle and bend CD thermoplastic fibers inthe present invention are given by a sine function of the fiber tensilestress and the angles (see FIGS. 4 & 5) involved become smaller as MDprocessing draw ratio increases, so compressive forces diminish withdraw ratio. In addition, the distribution of filament diameters in theabove precursor web is an order of magnitude larger than those of thepresent invention and thus the bending rigidity of CD thermoplasticfibers is very much higher while compression stresses are relativelysmall during processing. Elastomeric polymer webs (e.g., elastomershaving rubber-like properties of an elastomer or rubber; that is, havingthe ability to stretch at least twice their original length and retractat room temperature) cannot be used in the present invention.

The synthetic thermoplastic fibrous layers of the precursor nonwoven webmay be made from many of the man-made thermoplastics capable of beingmelt blown, provided the polymer selected develops filaments ofsufficiently high tensile processing modulus to permit the developmentof high lateral compression forces on the web. The thermoplastic resinsuseable in the production of nonwovens of man-made fibers include thenonelastomeric polyolefins such as polyethylene, polypropylene includinghigh density polyethylene, ethylene copolymers (including EVA and EMAcopolymers with high tensile moduli), nylon, polyamides, polyesters,polystyrene, poly-4-methylpentene-1, polymethylmethacrylate,polytrifluorochlorethylene, polyurethanes, polycarbonates, silicones,polyphenylene sulfide liquid crystal polymers, and fluoropolymers.

The crystallinity of the thermoplastic fibers of the precursor webpreferably should be sufficiently high to provide a room temperaturebreaking elongation of the precursor web of less than 40%. Meltblownwebs useable in the present invention should break at a strain of lessthan 40 percent in accordance with ASTM test method D 5035-90. Thecrystallinity in the range of 30 to 70 percent is preferred. In general,the proper high modulus and state of molecular orientation of theprecursor is best reflected by a maximum or breaking draw ratio of theweb during post treating of less than about 4.0.

In the post-treatment process, the thickness of the precursor web shouldpreferably be at least 2 mils and up to about 200 mils. The width of theweb, of course, can vary within wide limits, with 5 to 150 inches beingpreferred. The average fiber diameter of the precursor meltblown fibrouslayer of the web will preferably range from 0.5 to 8 microns, with 2 to6 microns being preferred in order to provide the proper range ofprocessing tensile stiffness for PP web. The porosity of the precursorweb will normally be in the range of 50 to 95 percent. Calenderedprecursor webs approach 50%.

Other properties of the web, which while not critical, are importantinclude a low occurrence of large shot or excessive ropiness.

Another important feature of the precursor web is that each of thelayers of man-made fibers includes at least some fiber-to-fiber bondingwhich is typical in meltblown and spunbonded webs. The bonding can beachieved by inherent fiber-to-fiber fusion, or by point bonding,calendering, or by fiber entanglement. The properties of the selectedpolymer can be controlled to a degree by operation of the meltblowing orspunbonded process. Some of these control variables are disclosed underthe experiments below.

Whereas the meltblown and spunbonded webs of thermoplastic man-madefibers of the prior art have required special and additional treatmentfollowing their formation in order to make these webs useful indisposable medical and sanitary products, the present inventors havefound that through the combination of selected ones of these webs withselected cellulose-based layers of natural fibers in a bondedconsolidated web, it is possible to produce a consolidated web whichdoes not require that the man-made fibrous webs be specially treatedindependently, but rather these selected webs can be directlyincorporated into the precursor web, hence into the consolidated web ofthe present invention. This capability provides the present inventionwith a substantial economic advantage.

As noted, a preferred precursor web in accordance with the presentinvention comprises an inner layer of cellulose-based natural fiberswhich is sandwiched between outer layers of man-made thermoplasticfibers. The precursor web, therefore, may comprise differentcombinations of layers. For example, in addition to the required layerof cellulose-based fibers, the precursor web may include a first layerof meltblown man-made fibers facing one surface of the cellulose fibersand a third layer comprising spunbonded man-made fibers facing theopposite surface of the cellulose fiber layer. In like manner, the firstand third layers may both be either meltblown or spunbonded fibers.Still further, there may be provided multiple layers of cellulose fiberswhich may or may not be separated by additional inner layers of man-madefibers, either meltblown or spunbonded. In any event, the cellulosefibers are to be protected by outer layers of man-made fibers. It willbe recognized that the addition of further layers to the precursor webincreases the cost of the web and may detract from the hand and otherdesirable properties of the consolidated web.

In FIG. 10 there is depicted a web 40 which includes thermoplasticlayers 42 and 44, plus a cellulose fiber layer 50 sandwichedtherebetween. As depicted, these layers are bonded one to the other by apattern of diamond-shaped bonds 46 which are each of substantially thesame size and spaced apart from each other.

In FIG. 11, there is depicted schematically a process, for overlayingpreviously formed layers 45, 47 and 49 into a web into a forwardlymoving conveyor 51 and thereafter bonding the layers into a coherent web40 by passing the web through the nip 54 of a set of heated rolls 56 and58. In this embodiment, the upper roll 58 is provided with a pattern ofsurface projections 60 which enhance the formation of the desired spacedapart bond areas 46. As depicted, the composite web 40 is collected in aroll 62 for storage and subsequent use. As desired, each of the webs 45and 49 is formed from man-made fibers, e.g., by spunbonding, meltblowingor other process which provide a coherent self-sustaining web.

In FIG. 12, there is depicted schematically a process for themanufacture of a web of the present invention in which a first layer 70of man-made thermoplastic fibers is formed employing a conventionalmeltblowing or spunbonding process 74 and thereafter deposited on aforwardly moving conveyor 72. A layer 78 of cellulose-based fibersproduced either offline or inline as described in FIG. 13, is overlaidonto the first layer 70 that is disposed on the moving conveyor 72. Athird layer 80 of thermoplastic man-made fibers is formed by aconventional meltblowing or spunbonding process 81 and overlaid onto thecellulose-based layer 78 to provide a three-layered web in which thecellulose-based fibrous layer 78 is disposed between outer layers 70 and80 of man-made thermoplastic fibrous material. In the depicted process,these several overlaid layers are fed through the nip of a set of heatedpressure rolls 84 and 86, one of which has a pattern of projections 88on its outer surface, to thermally bond the several layers into acoherent web 89. The composite web may be collected in a roll 92 forfurther use. As discussed herein, one or both of the first and thirdlayers, 70 and 80 may be formed by conventional meltblowing, spunbondingor like techniques, including thermal bonding of man-made staple fiberwebs.

With reference to FIG. 13, there is depicted a further embodiment of aprocess for the manufacture of a web in accordance with the presentinvention. In the depicted process, a first web 94 of man-made fibers isformed as by means of an on-line conventional melt-blowing orspunbonding apparatus 96, fed past an idler roller 95, and deposited onthe upper run of a first conveyor 97. As depicted, the process furtherincludes an in-line carding section 98 in which a bale 99 ofcellulose-based fibrous material is introduced to an in-line cardingunit 100 from which a carded web 101 is fed directly from the cardingunit onto a second conveyor 102. From the conveyor 102, the cellulosicweb is fed forwardly onto the top of the web 94 on the conveyor 97.Further, a third web 104 of man-made fibers is formed as by means of afurther in-line conventional meltblowing or spunbonding apparatus 105and fed past an idler roller 106, and overlaid upon the top surface ofthe cellulosic web 101 wherein the cellulosic layer 101 becomessandwiched between the webs 94 and 104 of man-made fibers. These layersof webs are fed forwardly through the nip 107 of a set of heated rolls108 and 109, the upper one 108 of which is provided with projections 111on its outer cylindrical surface for effecting spaced-apart thermalbonds between at least the top web 104 and the cellulosic web 101 toform the layers into a composite web. The bonded composite 113 iscollected in a roll 115 for subsequent storage and use. Optionally, alayer of man-made staple fibers may be formed into a web 117 as by meansof a conventional air laying web former 119 and interposed into thecomposite 113 between the cellulosic web 101 and one or both of theman-made fiber webs 94 and 104.

Samples of precursor webs employing features of the present inventionwere manufactured employing the process depicted schematically in FIG.11. In the preparation of the present samples, the cellulose-basedfibers were fed to an opener-mixer where the fibers from a bale wereopened and uniformly mixed. The fibers from the opener mixer were fedthrough a card wherein the fibers were carded to develop a web which wasdoffed directly from the card, without being wound up, and fed onto alayer of thermoplastic man-made fibers carried on a conveyor. The cardemployed in the manufacture of the present samples had a randomizingunit attached to its exit end so that the fibers were randomly orientedin the web with little or no preferred orientation in the machinedirection. Thereafter, a third layer, comprising thermoplastic man-madefibers, was overlaid on top of the cellulose fiber layer so that thecellulose fiber layer was sandwiched between the two outer layers ofthermoplastic man-made fibers. This laminate was then fed through thenip between a set of heated rolls, one of which was of a smooth surfaceand other of which was provided with a pattern of spaced projections,each of which was of a diamond-shaped cross section. Tables I and IIprovide further details regarding the operational parameters employed inthe preparation of these samples and the composition of the varioussamples.

                  TABLE I                                                         ______________________________________                                        Parameters and Their Levels                                                                No. of                                                           Parameter    Levels  Values                                                   ______________________________________                                        Melt Blown Fabric                                                             1.  Resin        2       Himont Valtec 442, Exxon                                                      PD 3495G                                             2.  Fabric Weight                                                                              2       0.7 oz/yd.sup.2, 0.5 oz/yd.sup.2                     Staple Fiber Web                                                              1.  Weight       1       1.0 oz/yd.sup.2                                      2.  Constituent Fibers                                                                         2       Cotton (C), Polypropylene (PP)                       3.  Fiber Denier                                                              Cotton Denier                                                                     1            1.75 (Veratec `Easy Street`)                                 PP Denier                                                                         2            2.2 (Hercules T-185)                                                                  3.0 (BASF bico `Merge 1080`)                         4.  Fiber Length                                                              Cotton Length                                                                     1            1.0 inch                                                     PP Length                                                                         1            1.5 inch                                                     Thermal Bonding Process                                                       1.  Pattern of   1       Diamond                                                  engraved roll                                                             2.  Area percent of                                                                            1       16.6%*                                                   raised pattern                                                            3.  Nip Pressure 1       250 PLI (pounds/linear inch)                         4.  Temperature:                                                              Top Roll                                                                          4            128° C., 133° C., 134° C.,                               135° C.                                               Bottom Roll                                                                       4            127° C., 129° C., 131° C.,                               132° C.                                               5.  Surface Speed                                                                              1       29 ft/min                                                of Calender Rolls                                                         ______________________________________                                         *Bonding area of Kusters Calender used to make samples in Table II       

                                      TABLE II                                    __________________________________________________________________________    PROCESSING CONDITIONS OF                                                      MELT BLOWN/COTTON/MELT BLOWN LAMINATE.sup.1 SAMPLES                               Weight of Layers.sup.2    Bonding Roll                                                                            Composition                           Sample                                                                            (oz/yd.sup.2)                                                                          Composition of Layers                                                                          Temperature (°C.)                                                                of Composite Web                      No. Top/Mid/Bottom                                                                         Top  Middle Bottom                                                                             Top  Bottom                                                                             Cotton (%)                                                                          PP (%)                          __________________________________________________________________________    1   0.7/1.0/0.7                                                                             UT-1-24.sup.3                                                                     100% Cotton                                                                          UT-1-24                                                                            128  129  41.8  58.2                            2   0.7/1.0/0.7                                                                            UT-1-24                                                                            100% Cotton                                                                          UT-1-24                                                                            134  129  41.8  58.2                            3   0.7/1.0/0.5                                                                            UT-1-24                                                                            100% Cotton                                                                           UT-1-17.sup.4                                                                     134  129  45.4  54.6                            4   0.7/1.0/0.7                                                                            UT-1-24                                                                            100% PP.sup.5                                                                        UT-1-24                                                                            135  132  0     100                             5   0.7/1/0/0.5                                                                            UT-1-24                                                                            100% PP.sup.5                                                                        UT-1-17                                                                            135  132  0     100                             6   0.7/1.0/0.5                                                                            UT-1-24                                                                            100% BF PP.sup.6                                                                     UT-1-17                                                                            135  132  0     100                             7   0.7/1.0/07                                                                             UT-1-24                                                                            100% BF PP.sup.6                                                                     UT-1-24                                                                            135  132  0     100                             __________________________________________________________________________     .sup.1 40inch webs produced.                                                  .sup.2 Outer layers consisted of different weights of meltblown (MB)          Polypropylene (PP) and middle layer consisted of staple fiber.                 .sup.3 Himont Resin MB Polypropylene (0.7 oz/yd.sup.2).                      .sup.4 Himont Resin MB Polypropylene (0.5 oz/yd.sup.2).                       .sup.5 Hercules Grade T185 Polypropylene.                                     .sup.6 BASF bicomponent fiber.                                           

The precursor web samples produced as listed in Tables I and II weretested for various properties as indicated below:

Barrier.

Barrier refers to the ability of a fabric to resist strike-through offluid and microorganisms. Barrier properties protect the operating roomstaff and the patient from infection.

    ______________________________________                                        Test            Test Procedure Used                                           ______________________________________                                        Hydrostatic Pressure                                                                          AATCC Test Method 127-1985                                    Oil Repellency Rating                                                                         AATCC Test Method 118-1983                                    Water Impact Penetration                                                                      AATCC Test Method 42-1985                                     Water Spray Rating                                                                            AATCC Test Method 22-1985                                     ______________________________________                                    

Strength.

Medical nonwovens also need to be strong enough to prevent tearing andpuncturing all the way from manufacturing steps through use of thefinished product.

    ______________________________________                                        Test                Test Procedure                                            ______________________________________                                        Breaking Load       IST.sup.1 110.0-70 (82)                                   Elmendorf Tear Strength                                                                           IST 100.0-70 (R82)                                        Mullen Bursting Strength                                                                          IST 30.0-70 (R82)                                         Tensile Elongation  IST 110.0-70 (82)                                         ______________________________________                                         .sup.1 INDA (Association of the Nonwovens Fabrics Industry) Standard Test                                                                              

Drapability and Comfort.

Drapability of a nonwoven fabric refers to its ability to conform to theshape of the object it is covering. The objects include patients,operating room tables and equipment.

Comfort relates to breathability, selection of materials and productdesign.

    ______________________________________                                        Test                 Test Procedure                                           ______________________________________                                        Frazier Air Permeability                                                                           IST 70.1-70 (R82)                                        Cantilever Bending Length                                                                          ASTM D 1388-64                                           ______________________________________                                    

The results of these tests are given in Table III.

                                      TABLE III                                   __________________________________________________________________________    TEST RESULTS OF UNFINISHED LAMINATE FABRICS                                   __________________________________________________________________________        Bending      Tear                  Breaking                                   Length                                                                              Bursting                                                                             Strength              Strength                                                                            Elongation                       Sample                                                                            (cms) Strength                                                                             (gms)   Air Permeability                                                                            (Kg/cm)                                                                             (%)                              No. MD CD (psi)                                                                            (kPa)                                                                             MD  CD  cu. ft/min/ft.sup.2                                                                  cu. m/sec/m.sup.2                                                                    MD CD MD CD                            __________________________________________________________________________    1   7.22                                                                             5.63                                                                             11 75.79                                                                              98 174 32.00  0.16   0.83                                                                             0.54                                                                             14 21.2                          2   7.91                                                                             5.97                                                                             9.4                                                                              64.77                                                                              84 126 30.51  0.16   0.90                                                                             0.49                                                                             11.6                                                                             20                            3   7.02                                                                             5.27                                                                             7.7                                                                              53.05                                                                              68 114 32.84  0.17   0.80                                                                             0.43                                                                             10.4                                                                             22.8                          4   7.4                                                                              5.14                                                                             19.1                                                                             131.60                                                                            158 694 30.90  0.16   0.97                                                                             0.50                                                                             20 24.8                          5   6.98                                                                             5.20                                                                             17.3                                                                             119.20                                                                            126 488 36.70  0.19   0.88                                                                             0.40                                                                             22.4                             6   7.37                                                                             5.14                                                                             19.4                                                                             133.67                                                                            166 248 36.42  0.19   1.46                                                                             0.45                                                                             24.8                                                                             24.4                          7   7.53                                                                             5.49                                                                             19.1                                                                             131.60                                                                            112 292 30.17  0.15   1.39                                                                             0.54                                                                             26 24                            8   3.68                                                                             4.04                                                                             39.5                                                                             272.16                                                                            853 1209                                                                              26.37  0.132  1.42                                                                             1.59                                                                             35.6                                                                             34.4                          9   3.93                                                                             2.70                                                                             39.3                                                                             270.78                                                                            613 660 16.77  0.083  1.49                                                                             1.35                                                                             22 28.4                          10  4.62                                                                             4.88                                                                             40.3                                                                             277.67                                                                            1179                                                                              1755                                                                              13.66  0.068  1.32                                                                             1.66                                                                             31.2                                                                             35.2                          11  3.90                                                                             2.94                                                                             42.5                                                                             292.83                                                                            641 746 11.9   0.059  1.59                                                                             1.33                                                                             24.8                                                                             27.6                          __________________________________________________________________________                           Hydrostatic  Water Impact                                                 Sample                                                                            Pressure                                                                            Water Spray                                                                          Penetration                                                                          Oil Repellency                                        No. (cm)  Rating (gm)   Rating                             __________________________________________________________________________                       1   34    90     0.43   0                                                     2   32    80     0.37   0                                                     3   24    70     0.9    0                                                     4   39    70     0.83   0                                                     5   57    80     4.33   0                                                     6   42    80     1.73   0                                                     7   48    70     0.33   0                                                     8   50    90     0      0                                                     9   62    70     0      8.0                                                   10  77    90     0      0                                                     11  58    70     0      7.5                                __________________________________________________________________________     Note:                                                                         Sample No. 8 = 1.8 oz/sq. yd unfinished SMS (Spunbonded/melt                  blown/spunbonded) fabric.                                                     Sample No. 9 = 1.8 oz/sq. yd finished SMS fabric.                             Sample No. 10 = 2.3 oz/sq. yd unfinished SMS fabric.                          Sample No. 11 = 2.3 oz/sq. yd finished SMS fabric.                       

As indicated above, the primary purpose of the process of the presentinvention is to consolidate the precursor web in the cross direction toreduce the average pore size and the pore size distribution in the weband to impart to the web enhanced breathability, strength, hand, drape,absorbent capacity and barrier properties. Consolidation of the web inthe cross-direction is to be distinguished from consolidation resultingfrom calendering since consolidation to reduce thickness as incalendering flattens the fibers and closes flow channels, thusdecreasing the permeability of the web to a greater extent compared toweb draw consolidation.

The random nature of low stretch meltblown webs with the attendantthermal bonding and/or filament entanglement enable the development ofMD stresses to reorient fibers and create sufficient compressivestresses to laterally consolidate or squeeze them together thus reducingthe size of voids there between during uniaxial drawing. This results innarrowing of the web width without disrupting the planar integrity ofthe web and produces a product of unique properties. During thepost-drawing process, the modulus that is in effect while the filamentsegments are being drawn depends on processing time-temperature effects.Maximum consolidation in the CD is achieved at a trial and error modulusat which the compressive stresses overcome to the largest extent thecritical buckling stresses for the population of CD segments in the web.This is illustrated in the post-drawing process preferably carried outat a temperature where the polymer is in the rubbery state. This is bestillustrated with reference to FIGS. 4, 5 and 6 which depict,respectively, the random disposition of nonwoven fiber, an idealizedrepresentation of unconsolidated nonwoven fibers, and an idealizedrepresentation of consolidated nonwoven fibers. The random dispositionof the filaments forming a thin planar layer of the meltblown web isrepresented in FIG. 4 wherein longitudinal fibers 26 extend generally inthe MD, transverse fibers 27 extended in the CD, and intermediatesegments of fibers 28 extend at an angle with respect to the MD and CD.

For purposes of analysis, this planar disposition may be represented byrepresentative cells illustrated in FIG. 5. In the idealizedrepresentation or model in FIG. 5, the fibers 26, 27, and 28 are showninterconnected or bonded as a loose network at junctions 29 of thefibers. Again, it is to be emphasized that the bonds are fuse bondedduring the meltblown process, or by fiber entanglement, or by thermalpoint calendering techniques. When the web structure shown in FIG. 5 issubjected to tension in the MD, the intermediate fibers 28 are easilyaligned in the MD thus reducing pore dimensions whereas the CD fibers 27tend to resist compression of the cell in which it is associated and maybuckle and bend as illustrated in FIG. 6. The result is that the lateralconsolidation of the precursor web in accordance with the presentinvention leaves pore space throughout the web layer which depends onthe extent to which CD fibers are buckled. Fiber having a highslenderness ratio of length by diameter buckle easier. Ideally, thecompressive force on element 27 in FIG. 6 is 2Tsin(theta) where T is thetensile force in elements 28 and Θ is the angle between element 28 andthe MD. Without the bonding at junctions 29, the webs would easilyrupture without generating lateral (CD) compression. Although actualwebs do not include only the idealized structure as depicted in FIGS. 5and 6, there is sufficient bonding and stresses developed in the selectprecursor web to provide the reduced porosity following the thermaldrawing process as in FIGS. 2 and 3. Note that the buckled CD fibers 27act as spacers limiting the residual porosity and pore dimensionsdeveloped by the resultant compression forces due to the MD tensiledrawing force. To supplement the compression of large diameter fibersand shot, external mechanical means can be incorporated to furthercompress the hot drawn web near 17c in order to augment the CD fiberbending and buckling beyond that obtained by hot drawing alone. One suchapparatus embodiment is illustrated in FIG. 3 described above in whichthe mostly drawn web is subjected to transverse compression forcesbecause the web is tracking the converging surface of the torus.

The post-drawn web withdrawn from the oven and preferably heat setexhibits several surprising and highly useful properties: (1) the porespace and all measures of pore size distribution have been reduced, (2)the web exhibits remarkable elasticity in the CD, and (3) the webexhibits enhanced strength, wicking, absorbent capacity, breathabilityand barrier properties. These properties will be discussed in detaillater.

The post-drawing process conditions and precursor properties forachieving the web with the improved properties described above are asfollows:

    ______________________________________                                                   BROAD   PREFERRED   BEST                                                      RANGE   RANGE       MODE                                           ______________________________________                                        Draw ratio, V2/V1                                                                          1.05-3.00 1.10-2.00    1.2-1.80                                  Temperature, °F.                                                                    165-425   250-350     275-300                                    (PP)                                                                          V1, Feed Speed,                                                                             10-400    25-200     35-60                                      F/M                                                                           MAX pore size, μM                                                                        5-250     10-150     20-50                                      Crystallinity, %                                                                           30-95     30-80       35-60                                      Thickness, mils                                                                             2-200     2-100       6-20                                      Avg. Fiber Dia. μM                                                                      0.5-50    0.5-8       1.7-6                                      Strain rate, per                                                                            10-500    20-200     30-60                                      min                                                                           Hot processing                                                                             1.3-4     1.7-3.5     2-3                                        breaking draw                                                                 ratio, V2/V1                                                                  Reduction in pore                                                                          20-85     25-75       35-70                                      size (MAX, MFP,                                                               and range), %                                                                 Elastic recovery                                                                           50-99     70-99       80-95                                      from 50% strain, %                                                            Liquid absorption                                                                          1.2-6     1.76-5      2-4                                        aspect ratio                                                                  ______________________________________                                    

It should be observed that the geometric minimum MD strain for completelateral consolidation of an idealized web in FIG. 5 is 42 percent orDR=1.42. However, in the most preferred embodiment the inventioncontemplates draw ratios in excess of about 1.42 since higher drawratios will enhance the reduction in porosity, which is limited by thespacer effects of partially buckled CD fibers.

Through the selection of the resin and meltblowing operating conditions,precursor webs having the necessary properties may be obtained basedupon the above description.

Although the man-made fibrous layers of the precursor webs made up ofany of the thermoplastic polymers used in meltblowing (provided theypossess the necessary properties) may be used, the followingpolypropylene meltblown layer of the precursor web has producedexcellent results in experiments carried out at the University ofTennessee.

    ______________________________________                                        PP Grade (Exxon Grade) PD-3495 G                                              MFR                    800                                                    Thickness              13 mil                                                 Width                  14 inches                                              Basis Weight           1.5 oz/yd.sup.2                                        Porosity               87%                                                    Crystallinity          50%                                                    Web elongation at break                                                                              10%                                                    ______________________________________                                    

As illustrated in FIG. 2, the precursor web 17 in a generally flatdisposition is processed according to the present invention by passingthe flat web 17 in an oven 23 at a temperature between the softening andmelting temperature of the polymer (e.g., for PP, about 310 degrees F.).The line speed and draw ratio are selected to impart the desired lateralconsolidation of the web expressed as a ratio of the webs 17a widthentering the oven to web 17c width exiting the oven (a/c in FIG. 2). Thea/c values should be from 1.3 to 4, preferably from 1.5 to 3, and mostpreferably 2 to 2.5. Web thickness entering the oven may range from 2mils to 100 mils and those exiting may range from between 2 and 150mils, indicating that the thickness may under certain conditionsincrease. Draw ratios of 1.05 to 3.00, preferably from 1.10 to 2.00, andmost preferably 1.2 to 1.8 may be used to achieve satisfactoryconsolidation. Line speeds (V2) can range from 10 to 400 fpm. Asmentioned above, webs capable of hot processing breaking draw ratiosgreater than about 4 are unsuitable.

It is preferred that the consolidated and annealed web leaving the ovenbe cooled, either by ambient temperature or supplemental air to impart aset to the fibers in the deformed condition. The consolidated heat setweb can be rolled up for later conversion to end use products.

The web consolidation restructures the thermoplastic fibers of the webby aligning more of the fibers in the MD. The fiber bonding transformstensile stress into CD consolidation in the manner described above,thereby reducing all of the web's measures of pore size distribution.These measures of pore size distribution of the web are the maximum poresize (MAX), the mean flow pore size (MFP), and the minimum pore size(MIN) as measured by a Coulter Porometer.

Definitions:

In order to better understand the terms used herein, particularly in theExperiments described below, the following definitions consistent withthe accepted technical definitions in the industry, are submitted.

Web Pore Space (porosity)--the ratio of the volume of air or voidcontained within the boundary of a material to the total volumeexpressed as a percentage. Packing density equals 1 minus porosity.

Coulter Porometer--a semiautomated instrument using a liquiddisplacement technique to measure the pore size measures anddistributions of a sample according to ASTM F 316-86.

Web Pore Size Distribution--the distribution of pore sizes between themaximum and the minimum pore size as determined by ASTM F 316-86 on theCoulter II Porometer. (The maximum pore size [or bubble point] measureis distinguished in that it strongly relates to permeability, pressuredrop, and filtration efficiency performance properties for the entirefamily of meltblown webs we studied.)

ASTM 316-86 Measures of Pore Size Distribution--MAX is the standardizedmeasure of the diameter of the largest pore channels in the distributionof pore sizes supporting flow through the web. MFP is the measure of themedian (or mean) pore channel diameter for the pores supporting thetotal flow. MIN is the minimum pore size measured for the web.

Polymer Crystallinity--the relative fraction of highly ordered molecularstructure regions compared to the poorly ordered amorphous regions.Crystallinity is determined by X-ray or DSC analysis.

Polymer Birefringence--is a property which is usually observed inoptical microscopes when a material is anisotropic, that is when itsrefractive index is directional. Fibers having molecular chains ofhigher axial directionality have higher birefringence and relatively lowtensile elongation at break.

Softening Temperature--is a thermal property of a polymer characterizedby a temperature at which the material becomes sticky, viscous, orelastic (soft) prior to melting and looses its room temperature modulus(and can undergo plastic elongation) leading to maximum molecularorientation and breakage.

Average Fiber Diameter--a measure of the mean fiber diameter of thefibers in the web obtained from individual measures of the fibersdiameters in focus on a scanning electron micrograph of the subjectfibrous web--about 100 fibers are measured. Finer fibers generally arisefrom greater draw-down in meltblowing and have higher birefringence.

Web Elongation at Break--for a crystalline polymer is strain rate andtemperature dependent. The elongation at break primarily measures theextent of a plastic deformation process beginning at the initial stateand terminating at the final well ordered state of molecular orientation(MO) of the polymer. Precursor webs having fibers of high crystallinityand order have low elongation to break (from R. J. Samuels, StructuredPolymer Properties, John Wiley & Sons, 1973). For the meltblown webs,evaluating the precursor MO state by breaking elongation is bestaccomplished at high temperatures instead of at standardized ASTM D5035-90 room temperature test because of the wide range in fiberdiameters, MO state and bonding in meltblown webs. The meltblownprecursor webs were characterized by the magnitude of the breaking drawratio attained while hot drawing at a strain rate at least 25 min-1 (or2500%/min) and temperature at least 10 F. below the melting temperatureof the precursor thermoplastic polymer (Hot breaking draw ratio).

Web Tensile Modulus--is the measure of the force required to produce asmall extension (or compression). A highly inextensible material willusually have a large modulus.

Web Elasticity--that property of a body by virtue of which it tends torecover its original size and shape after deformation. Elastic recoveryfrom elongation is given by (stretched length--recoveredlength)/(stretched length--original length). The recovery from aninitial elongation is stated, such as, a 47% recovery from a 100% CDstrain.

The process conditions to produce a desired meltblown sample forevaluation were controlled as follows:

(a) the level of hot-drawability, as related to birefringence andtensile modulus during processing is a function of fiber-diameter andwas controlled by varying the primary air levels in the line from 70 to95%,

(b) the level of bonding in the web was controlled by adjusting the airto polymer ratio, the die to collector distance, the air temperature,the melt temperature and collector vacuum. Tenacity and theelongation-at-break was used to qualify the bonding strength for thesamples.

The slenderness ratio of fiber segments subjected to compression as wellas the magnitude the bending forces developed by drawing are ultimatelyrelated to the above.

The post-drawing on the precursor web was done in experimental apparatussimilar to that illustrated in FIGS. 2 and 3. The rollers were providedwith speed controls.

The polymer used in all of the tests was polypropylene (PP). The PPprecursor web samples used in the tests are described in TABLE IV.

                                      TABLE IV                                    __________________________________________________________________________                      Pore Sz. Measures, μm                                        %  Packing                                                                            Ave. Fiber                                                                          Break         Break                                         Sample                                                                            Air                                                                              Density                                                                            Diam. μM                                                                         Elong.                                                                            Max MFP                                                                              Min                                                                              D.R.                                          __________________________________________________________________________    A   90 0.095                                                                              3.2   7.4 19.3                                                                              15.4                                                                             11.1                                                                             2.2                                           B   90 0.110                                                                              3.9   6.3 17.9                                                                              14.3                                                                             10.5                                                                             2.5                                           C   85 0.085                                                                              4.0   17.4                                                                              28.3                                                                              16.6                                                                             10.7                                                                             2.5                                           D   80 0.129                                                                              5.5   6.6 38.8                                                                              20.1                                                                             13.8                                                                             3.0                                           E   70 0.145                                                                              8.5   3.0 20.8                                                                              14.4                                                                             10.9                                                                             3.5                                           F   70 0.163                                                                              9.9   4.1 40.5                                                                              24.2                                                                             16.5                                                                             3.7                                           G   70 0.172                                                                              8.8   5.7 33.0                                                                              20.6                                                                             13.7                                                                             3.8                                           H   60 0.168                                                                              18.5  2.7 117.0                                                                             68.0                                                                             25.0                                                                             6.0                                           __________________________________________________________________________

Web Measurements:

Fiber diameters were measured by SEM photographs of the specimens.

Maximum, mean flow pore size, minimum, and pore size distribution spreadin terms of the maximum and minimum, was based on a Coulter Porometeraccording to ASTM F 316-86.

Pore Space:

Measurements were based on weights of dry specimens and the weight ofthe specimen wetted out with a liquid of known density. Planardensification is evidenced by the increase in packing density (PD)measure of the web given by the ratio of dry web weight to the weight ofthe void-free web. Porosity of the web or pore space is given by oneminus the packing density.

The tests for measuring elasticity of the consolidated web were asfollows: Measured the percentage to which specimen recovered itsoriginal (CD) length immediately following a given % (CD) elongation,for example, sample A recovered 92% of its original length following a100% CD elongation. Another test on the consolidated webs includeddirectional absorption of liquids. Surfactants for improving the waterwettability of the fibers were applied to PP webs prior to aqueousabsorption tests. The surfactants were nonionic and other types such asnonionic polyoxyethylenated tert-octylphenol, anionic ammonium laurylsulfate, and cationic sulfobetaines. Directional absorption wascharacterized by the aspects ratio of the absorption pattern producedwhen a milliliter of liquid was applied to a point on the specimensupported on a horizontal surface. For a variety of meltblown andspunbonded specimens, absorption aspect ratios ranged from 1.7 to about5. The test results carried out on the webs consolidated at a DR of 2are presented in TABLES V. The filtration efficiency values formeltblown webs variously consolidated at draw ratios of 1.0 (undrawnprecursor web) and 2 (precursor drawn 100% are plotted in FIG. 9.

                  TABLE V                                                         ______________________________________                                                        Properties of DR = 2.0,                                                       % of precursor web                                                  Oven    Elastic Recovery    Pore Size                                         Temp.   from strain of                                                                             Packing                                                                              Measures, μm                             Sample                                                                              °C.                                                                            50%      100%  Density                                                                              Max. MFP  Min.                            ______________________________________                                        A     150     95       92    214    50   46   42                              B     155     93       Break 250    44   39   39                              C     150     95       90    302    49   60   65                              D     150     95       90    163    38   48   51                              E     150     87       Break 124    155  124  118                             F     150     Break    Break 101    73   76   78                              G     150     85       Break  95    113  103  108                             H     150     Break    Break  99    128  115  --                              ______________________________________                                    

The Table V data and properties of webs consolidated at DR=2 reveal thatthe pore sizes of samples A through D were reduced by 38 to 65% and thepacking density for the same samples were increased from 163 to 302%.

The maximum hot draw ratio is the magnitude of the breaking draw ratioduring hot processing and solely defines the molecular orientationpresent in the filaments of the precursor melt blown webs. Webs of PPhaving a maximum DR greater than about 3.5 are not consolidatedaccording to the present invention. Compare pore measures in Table IVand the changes produced at a DR of 2.0 in Table V. The data on FIG. 9indicates that filtration efficiencies improve significantly for fiberdiameters less than 8 microns, particularly less than 6 microns, thepreferred and most preferred fiber sizes. Note that these small fibersizes further distinguish over U.S. Pat. No. 4,048,364.

ALTERNATIVE EMBODIMENTS

Spunbond Webs:

As indicated above, the principles embodied in the present inventionhave application with nonwoven webs of manmade fibers other thanmeltblown webs. For example, for spunbond webs which are characterizedas having an average filament diameters of 7 to 50 microns andelongation to break less than about 200% according to ASTM Test D5035-90. The spunbond webs are prepared by melt spinning a multiplicityof filaments molecularly oriented by plastic deformation draw-down anddepositing the same on a moving collector to form a random collection ofuniform filaments arranged similar to that depicted in FIG. 4. Thedeposited filaments are then bonded by mechanical entangling, needling,hot calendering or otherwise thermal bonding at a plurality of points toimpart integrity and strength to the spunbond material. It should benoted that bonding such as thermal or mechanical bonding is normallynecessary since the filaments are not typically fused or sufficientlyentangled upon being laid or deposited on the collector. For spunbondedprecursors, the bonding must be strong (such as high temperature pointbonding) in order to locally elongate, buckle, and bend the filamentsegments without spoiling the web integrity (see FIGS. 5 and 6) becausethe drawn filaments have relatively high tenacity and modulus. In pointbonding, the bond points and bonding pattern generally are as follows:The area of heated bonding points are 5 to 25% of the roll area and theshape of the raised points can be diamond shaped or a number of othershapes and point distributions.

The consolidation of the spunbond (SB) web in accordance with thepresent invention occurs as follows: Hot drawing the SB web createsreduction in the measures of pore size and creates CD elasticity becausethe tensile forces generate sufficient compressive forces to plasticallybuckle and bend CD segments of the filaments for inventive reduction ofpore measures. The elasticity in the CD direction is a result of elasticrecovery from bending of the well bonded network of strong filamentsarranged similar to that idealized in FIG. 6.

An example of the spunbond process was as follows: Spunbonded web was 1meter wide, 1 oz/sq. yd. produced from 35 MFR PP on a Reicofil machinebonded between 90° and 140° C. at the University of Tennessee. Oventemperature 315° F., draw ratio 1.20 output velocity (V2) 50 FPM.

Electrostatic Charged Webs:

Another variation contemplated by the present invention is to apply anelectrostatic charge to the consolidated web to improve its filtrationperformance. The charging in the production of electrets can be appliedby a variety of techniques described in the patent literature. See forexample U.S. Pat. No. 4,592,815, the disclosure of which is incorporatedherein by reference. It is anticipated that the higher packing densityof fiber in the hot consolidated webs results in an unusually higheffectiveness of electron and ion implantation in the web. As an exampleof the effect of charging consolidated samples on web FiltrationEfficiency (FE), a 1.0 oz/sq.yd. precursor meltblown sample had an FE of30%, the FE after only consolidating this web at a DR of 1.5 was 79%,and finally the FE after charging this consolidated web was 99.8%.

Several 40-inch wide meltblown polypropylene (PP) precursor webs wereprepared at weights of 0.25, 0.50 and 0.75 oz/sq yd at the AccurateProducts Company of Hillside, N.J., and 0.6 oz/sq yd spunbonded PP webwas produced at the University of Tennessee, Knoxville, Tenn. Tencotton-based nonwoven fabrics were produced at the John D. HollingsworthCompany , Greenville, S.C. at a width of 40 inches (trimmed to a widthof 36 inches). A Hollingsworth 40-inch card with flat tops and with aWeb Master Take-Off was utilized to produce a cotton (Veratec EasyStreet scoured and bleached cotton) core web with a weight of 1.0 oz/sqyd. Cotton core webs of 1.5, 2.0 and 3.0 oz/sq yd were produced byemploying a 2.5 meter Hollingsworth Master Card to feed the randomcarded cotton webs into a Hergeth Model 6.430 Crosslapper. These webswere lightly needled (125 punches/sq inch using one board) with a DiloModel ODR needle loom using a Foster needle (15×18×3 style). The webswhere transported to the 40 inch carding and thermal bonding line and aMB (or SB as the case called for) web was mounted at the rear of theconveyor system for the carded webs. The specified weight of cotton coreweb was unwound onto the MB (or SB) web travelling on the conveyorrunning under the card. As noted above, the 1.0 oz/sq yd webs werecarded on that card utilizing the Web Master Take-Off. The top MB (orSB) web was unwound onto the carded cotton web and then the tri-laminatewas conveyed at a 45 degree angle to the nip of a Kusters 2-Roll 1.5meter thermal calender with the top heated steel roll having a raiseddiamond pattern resulting in 16.6% bonded area. The bottom heated rollhad a smooth steel surface.

Table VI provides an identification of the tri-laminates, particularlytheir composition. Table VII provides a comparison of several of theproperties of the tri-laminates before consolidation and the values ofthese same properties after consolidation in the manner describedhereinbefore. Table VIII presents the results of absorbent capacity andretention capacity of the tri-laminates before and after consolidation.Table IX presents the results of testing of the consolidatedtri-laminates for elastic recovery, breaking strength and elongation.

                  TABLE VI                                                        ______________________________________                                        TRI-LAMINATE SAMPLE IDENTIFICATION                                                                  Weight of Layers                                                                            Total                                     Sample                                                                              Sample          (oz/yd.sup.2) Weight                                    No.   Description     Top/Middle/Bottom                                                                           g/m.sup.2                                 ______________________________________                                              MCM LAMINATES                                                            1C   MCM-42-2.4.sup.1                                                                              0.7/1.0/0.7   2.4                                        6C   MCM-67-4.5      0.75/3.0/0.75 4.5                                        7C   MCM-73-2.75     0.25/2.0/0.5  2.75                                            SCM LAMINATES                                                            9C   SCM-60-3.35.sup.2                                                                             0.60/2.0/0.75 3.35                                      10C   SCM-69-4.35     0.60/3.0/0.75 4.35                                            SCS LAMINATE                                                            11C   SCS-45-2.2.sup.3                                                                              0.60/1.0/0.60 2.2                                             REPELLENT FIN-                                                                ISHED LAMINATES                                                          1C-R.sup.4                                                                         MCM-40-2.5      0.75/1.0/0.75 2.5                                       10C-R SCM-69-4.35     0.60/3.0/0.75 4.35                                      ______________________________________                                         .sup.1 MCM42-2.4 represents a thermally bonded trilaminate fabric             consisting of a melt blow (MB) web on both sides of a 100% cotton core we     with a total cotton content of 42% and a nominal weight of 2.4                oz./yd.sup.2.                                                                 .sup.2 SCM60-3.35 represents a trilaminate fabric consisting of a spunbon     (SB) PP on the face side and MB PP on the back side with a total cotton       content of 60% and a nominal weight of 3.35 oz/yd.sup.2.                      .sup.3 SCS45-2.2 represents a trilaminate fabric consisting of a spunbond     (SB) PP on the face side and back side with a total cotton content of         45.4% and a nominal weight of 2.2 oz/yd.sup.2.                                .sup.4 Repellent finished laminate.                                      

                                      TABLE IIV                                   __________________________________________________________________________    PROPERTIES OF TRI-LAMINATE WEBS BEFORE AND AFTER CONSOLIDATION                __________________________________________________________________________        Basis   Air     Hydrostatic                                                                           Oil Repellency                                        Weight  Permeability                                                                          Pressure                                                                              Rating  Wicking Ratio                             Sample                                                                            (oz. yd.sup.2)                                                                        (ft.sup.3 /min. ft.sup.3)                                                             (inches)                                                                              (0-8)   (MD:CD Ratio)                             No. Before                                                                            After                                                                             Before                                                                            After                                                                             Before                                                                            After                                                                             Before                                                                            After                                                                             Before                                                                             After                                __________________________________________________________________________     1C 2.4 2.63                                                                              15.7                                                                              18.9                                                                              20.3                                                                              --  5   0   1:75:1.25                                                                          1:0.375                               6C 4.5 3.39                                                                              11.0                                                                              13.53                                                                             13.2                                                                              --  5   0   1.5:1.25                                                                           1:1                                   7C 2.75                                                                              2.58                                                                              --  29.76                                                                             --  --  5   0   1.25:1.0                                                                           0.75:1                                9C 3.35                                                                              3.07                                                                              20.7                                                                              29.23                                                                             10.5                                                                              --  5   0   1.5:1.25                                                                           1:1.5                                10C 4.35                                                                              3.39                                                                              20.6                                                                              29.06                                                                             11.0                                                                              --  5   0   1.0:1.0                                                                            0.75:1                               11C 2.2 2.23                                                                              212.2                                                                             235.50                                                                            4.3 --  5   0   1.25:1.0                                                                           1:0.75                                1C-R*                                                                            2.5 2.75                                                                              --  18.07                                                                             --  11.5                                                                              5   6   0:0  0:0                                  10C-R*                                                                            4.35                                                                              3.45                                                                              --  35.50                                                                             --   5.5                                                                              8   8   0:0  0:0                                  __________________________________________________________________________                           Web     Pore Size of                                                          Thickness                                                                             Consolidated                                                      Sample                                                                            (mm)    Laminates μm                                                   No. Before                                                                            After                                                                             Min.                                                                              Max.                                                                              Mean Flow                              __________________________________________________________________________                        1C 0.5606                                                                            0.8783                                                                            9.043                                                                             28.20                                                                             15.19                                                      6C 0.7908                                                                            1.4928                                                                            6.757                                                                             18.14                                                                             10.97                                                      7C 0.6218                                                                            1.0078                                                                            8.164                                                                             20.48                                                                             12.905                                                     9C 0.7477                                                                            1.2990                                                                            6.757                                                                             25.20                                                                             12.73                                                     10C 0.7913                                                                            1.4675                                                                            7.109                                                                             33.51                                                                             11.50                                                     11C 0.5929                                                                            0.8233                                                                            42.85                                                                             69.19                                                                             51.255                                                     1C-R*                                                                            0.6750                                                                            0.7450                                                                            7.812                                                                             22.78                                                                             13.17                                                     10C-R*                                                                            1.3340                                                                            1.3995                                                                            7.988                                                                             46.71                                                                             15.89                                  __________________________________________________________________________     -- = Test not performed                                                       * = water repellent treated                                              

                  TABLE VIII                                                      ______________________________________                                        ABSORBENT CAPACITY (ml) & RETENTION                                           CAPACITY (ml) OF TRI-LAMINATES BEFORE                                         AND AFTER CONSOLIDATION                                                             Absorbent Ca-                                                                              Retention    Retention                                     Sample                                                                              pacity (100 Pa).sup.1                                                                      Capacity (3 kPA).sup.2                                                                     Capacity (5 kPa).sup.3                        No.   Before   After   Before After Before After                              ______________________________________                                         1C   10       18      9.0    17.5  8.5    17.5                                6C   13       18      11.0   12.0  10.0   11.5                                7C   --       18      --     9.0   --     9.0                                 9C   9        16      7.0    12.0  7.0    11.0                               10C   7        19      6.5    11.5  6.0    10.5                               11C   6         8      5.0    6.5   5.0    6.0                                 1C-R --       --      --     --    --     --                                 10C-R --       --      --     --    --     --                                 ______________________________________                                         .sup.1 Determined by subtracting the amount of liquid drained into the        graduated cylinder at the end of 10 minutes from the original 100 ml          dosage.                                                                       .sup.2 Determined by subtracting the amount of liquid unabsorbed at 3 kPa     pressure from the absorption capacity.                                        .sup.3 Determined by subtracting the amount of liquid unabsorbed at 5 kPa     pressure from the absorption capacity.                                        -- = Test not performed                                                  

                                      TABLE XI                                    __________________________________________________________________________    ELASTIC RECOVERY AND STRENGTH                                                 TEST RESULTS OF CONSOLIDATED TRI-LAMINATES                                    Elastic Recovery                                                                             Breaking Strength                                                                             Elongation                                     (%) From       (mN/tex)        (%)                                            CD Strain of   MD      CD      MD      CD                                     Sample No.                                                                          25%  50% Before                                                                            After                                                                             Before                                                                            After                                                                             Before                                                                            After                                                                             Before                                                                            After                              __________________________________________________________________________     1C   86   79  8.1 11.1                                                                              4.0 2.3 9.5 9.8 16.9                                                                              67.3                                6C   83   --  2.6 8.3 1.3 0.9 9.8 6.6 20.3                                                                              43.5                                7C   70   67  6.0 4.7 0.8 0.8 9.9 6.9 24.0                                                                              37.1                                9C   82   --  5.4 7.6 2.3 1.5 32.0                                                                              14.2                                                                              31.2                                                                              55.2                               10C   88   78  5.6 10.9                                                                              2.3 1.6 25.5                                                                              17.2                                                                              30.4                                                                              86.4                               11C   87   77  9.5 14.2                                                                              4.6 5.7 51.2                                                                              30.9                                                                              76.7                                                                              105.6                               1C-R 88   78  6.0 13.8                                                                              3.6 1.3 13.7                                                                              5.7 18.5                                                                              45.2                               10C-R 83   65  6.4 12.0                                                                              2.1 0.9 32.5                                                                              5.7 33.5                                                                              49.6                               __________________________________________________________________________     -- = break prior to 50% elongation                                       

From Table IX, it will be recognized that consolidation of thetri-laminates generally increased the MD breaking strength of theconsolidated webs, and in most cases there was no material change in theCD breaking strength of the webs. Consolidation of the webs generallyreduced the percent elongation of the webs in the MD, but greatlyincreased the percent elongation in the CD of the webs.

The increase in air permeability and decrease in hydrostatic pressurevalues of the consolidated webs over the non-consolidated webs aspresented in Table VII show the improved ability of the presentconsolidated webs to permit the flow of vapor or gases through thethickness of the webs, hence is an indication of their enhancedbreathability. This observation, taken in combination with the wickingratio and oil repellency ratings of the webs before and afterconsolidation, show that the webs exhibit enhanced resistance topenetration of liquids (e.g. blood) and bacteria through the thicknessof the consolidated webs. As shown in Table VIII, the enhanced absorbentcapacity values of the consolidated webs over the non-consolidated webs,along with their respective retention capacities, is a measure of theability of the consolidated webs to absorb and retain liquids within theconsolidated webs, hence their ability to prevent strike-through ofliquids from one surface of the webs to their opposite surfaces (i.e.,the liquids are readily captured within the cellulosic core layer of thewebs).

From Table IX, it may also be seen that the consolidated webs exhibitedgood elastic recovery from CD strain, thereby making the webs of valuein the manufacture of a large variety of products wherein elasticity ofthe webs is of some concern, such as in disposable garments for medicalpersonnel, pillow cases, etc.

The bending lengths of the tri-laminates were generally within a rangeof 1-2 cm, with the tri-laminates containing one or two layers of SBgenerally having lower bending lengths. Likewise, tri-laminatescontaining a SB layer appeared to have higher air permeability values.Sample IIC which contained two SB layers (SCS) had approximately tentimes greater air permeability than did the MCM and SCM tri-laminates.

Visual and physical examination of the consolidated webs containing theinner layer of cellulosic fibers showed their excellent hand. Upon suchexamination, the webs exhibited a soft feel to the touch and did notexhibit the characteristic roughness often experienced with MB or SBwebs or combinations of MB and/or SB webs. Whereas it is not known withcertainty, it is believed that the consolidation of the tri-laminateshaving an inner core web of cotton causes the cotton fibers to assume amore random orientation, both within the plane of the web and at anglesto the plane of the web, thereby causing the cotton fibers to impart anapparent bulkiness and softness to the overall web. In any event, theconsolidated tri-laminates exhibit a definitely enhanced hand ascompared to non-consolidated tri-laminates. Further, the consolidatedwebs exhibit good conformability.

The presence of the cellulosic layer in the consolidated webs of thepresent invention also makes the present webs more environmentallydesirable in that the cellulose fibers degrade relatively rapidly. In sodoing, the degradation tends to promote disintegration of the polymericcomponents of the webs, thereby making the present webs less deleteriousto the environment.

As demonstrated by the experimental data herein, the method of thepresent invention produces a nonwoven fabric that possesses unique anduseful properties that lend the fabric to application in a variety offields. The properties of reduced pore size and pore size distributionmakes the web ideally suited for filtration and absorption. The propertyof CD elasticity increases the web utility in filtration (e.g., surgicalmasks where conformance to the face contours is important) and otheruses such as flexible gowns or diapers and hygiene products. Theproperty of strength enhances the usefulness of the consolidated web inmost all applications.

What is claimed is:
 1. A planar consolidated multilayered nonwoven webwhich is consolidated in the cross-direction having a reduced maximumpore size which is at least 20% less than that of the web prior toconsolidation, wherein the web comprises first and third layers whicheach comprise a layer of randomly organized, nonelastomericthermoplastic man-made fibers bonded to each other, said thermoplasticfibers having a crystallinity prior to consolidation of at least 30%,and a second layer, sandwiched between said first and third layers,which second layer comprises non-wood cellulose-based staple fibers, andwherein in at least one of said first or third layers a majority of thefibers are aligned generally in the direction of draw and a minority ofthe fibers are disposed in a cross-direction transverse to the directionof draw.
 2. The consolidated web of claim 1 wherein the fibers are atleast partially coated with a surfactant for increasing the waterwettability of the web.
 3. The consolidated web of claim 1 wherein theconsolidated web has a mean flow pore size of between 3 and 40 μm. 4.The consolidated web of claim 3 wherein the consolidated web has anelasticity in the cross-direction of at least 70% recovery from a 50%elongation in the cross-direction.
 5. The consolidated web of claim 4wherein at least one of said first and third layers is a meltblown web.6. The consolidated web of claim 4 wherein both of said first and thirdlayers are meltblown webs.
 7. The consolidated web of claim 4 wherein atleast one of said first and third layers is a spunbonded web.
 8. Theconsolidated web of claim 7 wherein the staple fibers of said secondlayer comprise cotton.
 9. The consolidated web of claim 4 wherein bothof said first and third layers are spunbonded webs.
 10. The consolidatedweb of claim 4 wherein said first layer is a meltblown web and saidthird layer is a spunbonded web.
 11. The consolidated web of claim 4wherein the staple fibers of said second layer comprise cotton.
 12. Theconsolidated web of claim 4 wherein said first and third layers aremeltblown webs, and the staple fibers of said second layer comprisecotton.
 13. The composite web of claim 4 wherein the web has anelectrostatic charge applied thereto.
 14. A filter comprising theconsolidated web of claim
 1. 15. A face mask comprising the filter ofclaim
 14. 16. The composite web of claim 1 wherein the fibers contain anelectrostatic charge.
 17. The consolidated web of claim 1 which furthercomprises, positioned between the first and third layers, an additionallayer of non-wood cellulose based staple fibers.
 18. The consolidatedweb of claim 17 which further comprises an additional layer of man-madefibers separating the layers of non-wood cellulose based staple fibers.19. A method for forming a planar consolidated multilayered nonwoven webwhich comprises at least one thermoplastic component, which methodcomprises concurrently heating a nonwoven precursor web to a temperaturebetween the softening temperature and melting temperature of at leastone of the thermoplastic components and drawing the web in asubstantially longitudinal direction which web comprises first and thirdlayers which each comprise a layer of randomly organized, nonelastomericthermoplastic man-made fibers bonded to each other, said thermoplasticfibers having a crystallinity of at least 30%, and a second layer,sandwiched between said first and third layers, which second layercomprises non-wood cellulose-based staple fibers, wherein, in at leastone of said first or third layers, a majority of the fibers are alignedgenerally in the direction of draw and a minority of the fibers aredisposed in a cross-direction transverse to the direction of draw, andforming the consolidated web which has a reduced maximum pore size by atleast 20% than that of the web prior to consolidation.
 20. The method ofclaim 19 wherein the drawing step is sufficient to provide the web witha mean flow pore size which is at least 20% less than that of theprecursor web.
 21. The method of claim 19 wherein the drawing issufficient to provide the web with a packing density which is at least20% greater than that of the precursor web.
 22. The method of claim 19wherein the precursor web has a room temperature elongation at break ofless than 40% based on ASTM D 5035-90.
 23. The method of claim 19further comprising cooling the consolidated web or permitting the web tobe cooled.
 24. The method of claim 19 wherein at least one of said firstand third layers is a meltblown web.
 25. The method of claim 24 whereinthe precursor meltblown web has an average fiber diameter of 0.5 to 20μm.
 26. The method of claim 25 wherein the precursor meltblown web hasan average fiber diameter of 0.5 to 8.0 μm.
 27. The method of claim 24wherein the meltblown layer has a breaking elongation less than 40%based on ASTM D 5035-90.
 28. The method of claim 19 wherein both of saidfirst and third layers are meltblown webs.
 29. The method of claim 19wherein at least one of said first and second layers of the nonwovenprecursor web is a meltblown web having an average fiber diameter of 0.5to 8 microns and having an elongation at break of less than 30% based onASTM D 5035-90.
 30. The method of claim 19 wherein at least one of saidfirst and third layers is a spunbonded web.
 31. The method of claim 30wherein said spunbonded web has an average fiber diameter of 7 to 50 μmprior to consolidation.
 32. The method of claim 19 wherein both of saidfirst and third layers are spunbonded webs.
 33. The method of claim 19wherein said first layer is a meltblown web and said third layer is aspunbonded web.
 34. The method of claim 19 wherein the staple fiber ofsaid second layer comprise cotton.
 35. The method of claim 19 whereinsaid first and third layers are meltblown webs, and the staple fibers ofsaid second layer comprise cotton.
 36. The method of claim 19 whereinthe layers are thermally bonded together at spaced apart locations priorto consolidation.
 37. The method of claim 19 wherein the-thermoplasticin at least one of said first and third layers is selected from thegroup consisting of polyolefins nylon, polyesters, polyamides, cellulosetriacetate, cellulose diacetate, poly-4-methylpentene-1, polyphenylenesulfide, liquid crystal polymers and fluoropolymers.
 38. The method ofclaim 19 wherein the thermoplastic in at least one of said first andthird layers is a polyolefin selected from the group consisting ofpolypropylene, polyethylene, and copolymers thereof, and the heating iscarried out at a temperature of between 190° and 350° F.
 39. The methodof claim 19 further comprising passing the drawn web over a bar orroller having a surface to impart transverse compression forces inwardlyof the width of the web.
 40. The method of claim 19 wherein the heatingand drawing comprise passing the precursor web into an oven at a firstlinear velocity and withdrawing it from the oven at a second linearvelocity which is higher than the first velocity.
 41. The method ofclaim 40 wherein the first velocity is controlled by passing the webthrough the nip of first counter rotating rollers prior to the heatingstep, and the second velocity is controlled by passing the consolidatedweb through the nip of second counter rotating rollers after the heatingstep, and wherein the consolidated web is permitted to cool to atemperature below the softening point of the thermoplastics in said webprior to passing through the nip of said second rollers.
 42. The methodof claim 41, further comprising the step of passing the drawn web over abar or roller having a surface to impart transverse compression forcesinwardly of the width of the web.
 43. The method of claim 19 wherein theconsolidated web has a width of less than 80% of that of the precursorweb, and the ratio of the thickness of the consolidated web to that ofthe precursor web ranges from 1:1 to 1.5:1.
 44. The method of claim 19further comprising the step of applying an electrostatic charge to theconsolidated web.
 45. The method of claim 19 which comprises cooling theweb or permitting the web to cool.